Thanks for all the ideas, will have a look at those plastic "mini shed" type things. Have thought about the roof box, wasn't sure if it would be mouse proof (I believe they only need a gap of 6mm to get in).We've got a trailer but that is definately not mouse proof. Don't want my tent eaten!
Thanks for the argos link, the £29.99 one is a good price.
As for using the tent all the time....my son has requested that we camp in the snow ("so we can throw lots of snowballss") so it could well be a possibility!
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.